Electric bicycles are a form of dual-powered vehicles: they employ both a manual pedal and crank drive and an electric motor. These two drives may function independently of one another or may function together to augment one another's motive force. A user may choose to selectively engage the electric drive, or the electric drive may be activated automatically depending on such conditions as the measured pedal velocity, bicycle velocity, etc.
The electric drive may be located in several places; it may drive and be located within the hub of the rear wheel; it can power the pedal crank; or it may be located at some point between these two extremes, driving the chain of the bicycle. An alternative is to drive the front wheel, but this brings its own drawbacks.
The power source, usually a rechargeable battery, has to be located on the bicycle, and usually a bulky battery will be placed over or around the rear wheel.
Laws are in place around the world to limit the speed at which the electrical drive may propel such a bicycle, primarily for the safety of the user. The speed may be limited to around 15 mph. However, the user may be free to manually propel the bicycle beyond this velocity.
Drawbacks of current electrical bicycles include the bulk of the drive/battery mechanism making the bicycle cumbersome for the rider. A further drawback is in the potential for crank-driven pedalling or sudden cessation to damage the motor if that drives the crank. For example, an electric bicycle may be travelling under combined electric drive and user pedalling. If the user has to undertake an emergency stop, their reaction is to immediately stop pedalling holding the crank at a fixed angle. Whilst the bicycle may be provided with a brake lever mounted electric drive cut-off, the cessation of pedalling by the user may occur before this is activated and there will be a short period of time where the electric motor is driving the crank while the user is attempting to simultaneously hold the crank static. This can lead to the motor being damaged and/or the user's feet being forced around in an unwanted, unsettling and perhaps unbalancing pedalling motion.